> If I took the time to contact all the people I passed on to give them an explanation, it would consume more time than I can afford.

Maybe so, but a mail merge to say something to the effect of,

'Dear X,

Thankyou for taking the time to interview. On this occassion another candidate was a better fit, but we invite you to reapply if you see something you like in the future and wish you the best of luck finding suitable employment.

Kind regards, Y'

Takes a few minutes.